Hi. was he "moody" before this, before diabetes? If he is not on meds...sure being unwell, unsure, up and down, can have an impact on mood; as can people "encouraging" you (even lovely well-meaning people).
Questions -
How long has he been diabetic?
Is he regularly testing?
Is he frustrated with meter readings?
What do you mean by diet-controlled? Coz it sure don't sound too controlled to me...do you just mean no meds?
What exactly is your/his understanding of a "healthy" diet for a Type 2?
Solutions -
Only say well done when he does choose to eat sensibly (ie. low carbs)
Don't criticize or "advise" when he doesn't
Eat the same foods you want him to eat
Make up a "menu"/list together
Count the calories, carbs, sugar, fibre, protein content of each food (try to eat those with less than 10g carb per 100g on packet)
Record meter levels to demonstrate how well he can do when he eats the right stuff
Have this (ie the above) kind of talk with him ONLY after he has done something well and try to agree a plan
If all else fails tell him it's harder to walk with no toes (sorry - I am not being serious at this point...much)
Good luck